Microstructures of feldspar crystals from Pakra Creek valley granite (Papuk Mt., Croatia)

Optical polarizing microscopy, X-ray powder diffraction and transmission electron micro-scopy with energy-dispersive spectroscopy have been used to study microstructures in feldspars from two structurally and texturally different granite types from Pakra Creek valley (Papuk Mt., Croatia). The porphyritic texture granite contains pink potassium feldspar mega-crysts with cross-hatched extinction and tweed pattern, which proved to be intermediate microcline to orthoclase. These crystals have Or > 90% and An=0 composition, and coexist with low albite Or(1.2)Ab(98.8)An(0). The feldspars from the granite with homogenous fine-grained microstructure are white in colour and proved to be mixture of lamellar and zoned plagioclase and intermediate microcline. The plagioclase has a Na:Ca ratio close to 3:1 and minor K content (72.2%<Ab<76.5%, 23%<An<26.8%, Or<1.4%). Pure albite (Ab=97-99%) and anorthite (An=99%) grains were also detected. The higher content of plagioclase in the granite with homogenous fine-grained microstructure indicates a higher crystallization temperature compared to the porphyritic type. The microstructure of the feldspar crystals suggests a parent melt close to the eutectic composition for the studied granites.